There are a variety of situations in which it is desired to expand a metal tube radially to form a tight, leak-free joint. For example, large heat exchangers, particularly the type used as steam generators in nuclear power plants, often employ a tube sheet, which is a metal plate several feet in thickness through which hundreds of stainless steel or carbon steel tubes must pass. The tube sheet is initially fabricated with holes of a suitable diameter in which the tubes are inserted. The tubes are then expanded against the sides of the holes by plastic deformation to seal the small crevices that would otherwise exist around the tubes. If these crevices were allowed to remain, they could collect corrosive agents, and would, therefore, decrease the predictable life-expectancy of the equipment.
The traditional technique for expanding tubes radially within the holes of tube sheets employs mechanical rolling. There are, however, a number of significant disadvantages associate with this technique. For example, mechanical rolling causes elongation of the tube with an accompanying decrease in the thickness of the tube walls. In addition, it is a time consuming process that is difficult to employ in the case of longer tubes. The use of rolling also imposes a minimum dimension on the inside diameter of the tube in relation to the tube wall thickness, since it must be possible to insert rollers of suitable strength and rigidity.
For the above reasons, efforts have been made to develop techniques for expanding tubes by the application of fluid pressure. According to this newer technique, a mandrel is inserted in the tube and a pressurized working fluid is introduced through the mandrel into a small annular space between the mandrel and the tube. Fluid must be confined within the tube between two seals that surround the mandrel.
It has been found that the most effective seal consists of an O-ring, which interfaces directly with the working fluid, and a more rigid but still elasticity deformable back-up member behind the O-ring. As the back-up member is compressed axially, it expands radially against the inside of the tube.
It is necessary to find a material for this back-up member that has the necessary combination of hardness and elasticity, but does not deform plastically under high pressure. When plastic deformation takes place, it is often because the gap, the annular space between the mandrel and the tube, is too large, permitting a portion of the back-up member to be extruded into the gap. For this reason the gap between the mandrel and the tube is referred herein as the "extrusion gap."
It is generally possible, working with tolerances that are acceptable in this type of apparatus, to maintain an extrusion gap within satisfactory dimensional limits, provided that the gap is substantially uniform about the circumference of the tube. However, the mandrel tends to be positioned along the surface of the tube, thus producing a gap of double thickness at the top of the mandrel. It is in this area of double thickness that plastic deformation of the back-up member is generally found to occur.
It is an objective of the present invention to provide an improved sealing device that causes the extrusion gap to be substantially uniform, thereby minimizing problems of plastic deformation of sealing members.